A liquid crystal display device has some particular features such as the thin and light-weighted body, low power consumption, and has been widely used not only for a large display of television, personal computer etc. but also for a compact display of camera, portable movie camera, mobile phone, mobile terminal etc. A liquid crystal panel uses a polycrystal silicon (p-Si) thin film transistor (TFT) as a transistor for pixel driving, and is capable of integrating periphery drive circuits made of p-SiTFT outside the display region. Particularly, a low temperature polycrystal silicon thin film transistor substrate using an inexpensive glass substrate can be easily enlarged in size, and therefore has been recently used for an organic EL device in addition to its general usage for a liquid crystal display device.
A liquid crystal display device has various drive circuits such as a display controller, shift register etc., which are preferably capable of high speed driving, output buffer, level shifter, analog switch etc., which preferably have high pressure resistances. If the high speed driving is the first priority, the TFT preferably has a short channel length and does not include a LDD structure. On the other hand, if the high pressure resistance is indispensable, a desirable speed may not be obtained. For the TFT for pixel driving, the high pressure resistance is more important factor than high speed driving. A high pressure resistance TFT is required to endure a certain arbitrary level of high voltage, and therefore is preferably formed with a LDD or a gate insulating film having a sufficient thickness.
A pixel driving TFT is required to have a function of writing a data voltage when the gate voltage is on, and also a function of holding the written data voltage until the next data voltage is written. Therefore, it is preferable to reduce the leak current to a possible maximum degree when the gate voltage is off. To reduce the leak current, it is preferable to provide a lightly doped drain (LDD) between the channel of p-SiTFT and a low-resistance (highly doped) source/drain region. The pixel transistor is formed of an n-channel TFT (NTFT) having a higher performance than a p-channel TFT (PTFT).
A liquid crystal display device has some periphery circuits including an input-output circuit, which has a high pressure resistance, and a logic circuit. The logic circuit such as a shift register is preferably driven at a high speed. Therefore, two kinds of periphery p-SiTFT: a high pressure resistance transistor and a high speed transistor are required. Further, the periphery circuits are constituted of complementary MOS (CMOS) TFTs including a NTFT and a PTFT.
For a display of a mobile terminal or other small devices, reduction of power consumption is indispensable. Though a display section and some other parts need to be driven by a high voltage of 7V to 10V, a logic circuit section is preferably driven by a possible lowest voltage. To ensure the pressure resistance in the display section, the gate insulation film is required to have a thickness of 80-150 nm, but this spec is not suitable for the drive circuit section. Specifically, a drive circuit of this spec needs to be driven by a high voltage, thereby increasing power consumption. Low voltage driving is necessary for the logic circuit section to reduce power consumption; that is, two transistors are required: a transistor with a thick gate insulation film and a transistor with a thin gate insulating film.
Japanese Laid-Open Patent Application Tokukaihei 07-249778 (published on Sep. 26, 1995) teaches manufacturing of two kinds of transistors: a top-gate transistor and a bottom-gate transistor. In this structure with two gate transistors, the leak current decreases by offsetting the drain from the gate electrodes.
In a general method of polycrystallization of an amorphous silicon film by excimer laser, the thickness of film is limited. Therefore, when polycrystallization is performed with excimer laser after forming a gate insulating film and an amorphous silicon film, which are deposited on a bottom gate, the end section of the bottom gate electrode cannot be fully crystallized.
Japanese Laid-Open Patent Application Tokukai 2003-45892 (published on Feb. 14, 2003) teaches stacking a gate insulating film on a polycrystallization silicon film, forming a plurality of top-gate thin film transistors each having a gate insulating film of a different thickness. This invention suggests forming a lightly doped drain (LDD) region in the drain for a high pressure resistance n-type TFT, and forming a LDD region both in the drain and source for a pixel transistor.
According to the description (e.g. in the section of “Prior Art”) of Japanese Laid-Open Patent Application Tokukai 2000-299469 (published on Oct. 24, 2000), the OFF current of p-SiTFT tends to increase, while the ON current tends to decrease due to the hot-carrier effect. More specifically, hot-carrier generated by a high electric field in the vicinity of the drain is trapped in the gate insulating film, thereby decreasing the ON current. A GOLD (gate-drain overlapped LDD) structure, in which the gate electrode is overlapped with the LDD region, has an effect of reducing the high electric field in the vicinity of the drain, by which the hot-carrier effect also decreases. To form a LDD region beneath the gate electrode, it is necessary to form the LDD region with a mask before forming the gate electrode.
In recent years, lateral crystallization using continuous wave (CW) laser has come to the front as a replacement of excimer laser. Though crystallization using CW laser has been conventionally suggested, it has not been able to ensure a certain crystallization effect because of variation of laser light, more specifically, an excessive laser power density with respect to the energy for crystallization causes the film to be aggregated.
Japanese Laid-Open Patent Application Tokukai 2003-86505 (published on Mar. 20, 2003) teaches a polycrystallization method in which an amorphous semiconductor layer is first patterned into an island shape, and then is irradiated with continuous wave (CW) laser light from the rear surface of the transparent substrate. The method uses a solid state laser (DPSS laser) for semiconductor (LD) excitation. This method has an effect of increasing the size of crystal.
Scanning of an amorphous silicon film, which has been shaped into a several tens to several hundreds μm thick layer, by CW laser beam results in generation of a polycrystalline silicon film along the scanning direction, with an average grain diameter of several μm or greater. This irradiation is spot irradiation, which takes a significantly long time. The irradiation time may be reduced by performing polycrystallization by irradiation of only the element (such as TFT) forming region. For example, in forming a ribbon-shaped amorphous silicon film, a marker is formed with the ribbon, and only the TFT forming region is crystallized with the marker. Further, in the display region, only the pixel transistor forming region may be subjected to laser irradiation, which further reduces the irradiation time. The irradiation may be carried out by using a plurality of laser beams which are emitted simultaneously.
In recent technologies, the condition in the pre-processing of crystallization and in the laser irradiation are optimized, and therefore polycrystallization is carried out without forming a ribbon, causing no film removal. In the case where the amorphous film is not patterned, it is necessary to form the marker in advance. Compared with crystallization by excimer laser, this method with an extra marker forming process is lower in productivity.